Method and drying section for dewatering a fibrous web

ABSTRACT

The invention relates to a method and a drying section for dewatering a fibrous web ( 9 ), the drying section preferably being intended to dry fibrous webs in a paper machine, and the drying section comprising a number of drying cylinders ( 8 ), by which an inlet and outlet nip are formed when running the fibrous web, and the drying section comprising drying devices for decreasing the moisture content of the fibrous web, the drying devices comprising a blow device for blowing air in the proximity of the surface of the fibrous web. For the purpose of significantly increasing the speed of the paper machine or other machine and to be able to reduce the number of drying cylinders ( 8 ), and to reduce construction and operating costs upon dewatering, the drying section is characterized in that the drying devices comprise a porous drying cylinder ( 8 ) against which the fibrous web ( 9 ) is arranged to be guided wirelessly, a heating means ( 12 ) arranged at a distance from the shell surface of the porous drying cylinder ( 8 ) and guided against the fibrous web ( 9 ) and drying cylinder, for heating the fibrous web, the heating means being based on heating with rays having a wavelength which activates water molecules, and a blow device ( 15 ) comprising lateral blow devices arranged to provide a humid airflow which sweeps lengthwise along the surface of the fibrous web ( 9 ).

BACKGROUND OF THE INVENTION

The invention relates to a method of dewatering a fibrous web in adrying section, preferably a paper web, cellulose web or cardboard webin a paper machine, the drying section comprising a number of dryingcylinders, by which an inlet and outlet nip are formed when running thefibrous web, the drying section comprising drying devices for decreasingthe moisture content of the fibrous web.

The invention also relates to a drying section for dewatering a fibrousweb, the drying section preferably being intended to dry fibrous webs ina paper machine, such as paper webs, cellulose webs or cardboard webs,and the drying section comprising a number of drying cylinders, by whichan inlet and outlet nip are formed when running the fibrous web, and ablow device for blowing air in the proximity of the surface of thefibrous web.

A drying section is an essential part of a paper machine. The dryingsection serves to decrease the initially high moisture content of apaper web (or other fibrous web) so that its dry content increases toabout 90 to 97% when it leaves the drying section. Paper machines havebecome increasingly fast, and this has led to the need to make theirdrying sections correspondingly longer. In practice, this means that thedrying section may be a voluminous part of the paper machine having aconsiderable length, for example 80 meters, and comprising a pluralityof drying cylinders.

It is naturally desirable to accomplish a drying section so as tofurther increase the travel speed of the fibrous web, i.e. the speed ofthe paper machine, from what is customary, without making the dryingsection too complicated and large. Blowing hot air towards a paper webis a means for decreasing the length of the drying section. In spite ofthis, the drying section is a very complicated construction.

In this context, the concepts ‘humid air’ and ‘air’ refer to acombination of dry air and water vapour.

BRIEF DESCRIPTION OF THE INVENTION

The object of the present invention is to provide a significantimprovement to the drying section in machines processing fibrous webs,paper machines, for example, in such a way that the speed of themachines can be considerably increased and, simultaneously, the numberof drying cylinders can be decreased and the construction and operatingcosts lowered.

This object is achieved by the method of the invention, characterized byguiding the fibrous web wirelessly against a porous drying cylinder,heating the fibrous web by radiation heat directed towards the fibrousweb and the drying cylinder, and providing by means of lateral blowing ahumid airflow which sweeps in the machine direction along the surface ofthe fibrous web.

This object is achieved by the drying section of the invention,characterized in that the dewatering devices comprise

a porous drying cylinder against which the fibrous web is arranged to beguided wirelessly,

a heating means arranged at a distance from the shell surface of theporous drying cylinder and directed towards the fibrous web and dryingcylinder, for heating the fibrous web, the heating means being based onheating with rays having a wavelength which activates water molecules,and

the blow device comprising lateral blow devices arranged to provide ahumid airflow which sweeps along the surface of the fibrous web.

The heating employs electromagnetic waves, preferably IR rays (infraredrays) or microwaves.

The lateral blow devices are preferably arranged to blow in a directionopposite to that of the movement of the fibrous web, resulting in morerapid dewatering.

When the heating means is of a type based on gas energy, and the flameheat is guided without hindrance against the fibrous web, particularlygood heat transfer and a dewatering process having a high efficiency areachieved. Guiding heat without hindrance means herein that a heatabsorbing material (for example a ceramic material) is only partiallypresent between the fibrous web and the flame to achieve the desiredwavelength spectrum which thus allows optimal heat distribution andefficiency.

To further accelerate the dewatering process, blow devices arepreferably used for blowing humid air along the inside of the dryingcylinder, the air pressure being held lower than the air pressure actingabove the fibrous web. This allows moisture to be conveyed from thefibrous web through the drying cylinder, too. This embodiment enablesmoisture removal from both sides of the fibrous web. Part of thedischarge air can also be used to heat the inlet air.

Preferred embodiments of the invention are disclosed in the attachedclaims 3 to 16.

The use of heating means based on heating by electromagnetic waves, forinstance infrared rays, allows the surface temperature of a fibrous webto be increased even though the fibrous web is conveyed at a high speed,for example 20 to 40 m/s. The blow devices allow the moisture to befreed from the fibrous web and discharged rapidly.

The most important advantage of the invention is a more rapid dewateringprocess, which can be achieved at the same time as the construction andoperating costs can be kept lower than usual. The number of dryingcylinders can be substantially reduced, to about ⅕ of the number ofdrying cylinders in conventional cylinder drying. The costs per kilo ofevaporated water are substantially reduced, compared with conventionalpaper machines, by 20 to 40%. The heat energy based on electromagneticwaves can be transferred effectively, while the remaining heat energy istransferred parallel to the radiation energy in the form of forcedconvection and guidance through the porous cylinder. The drying sectionof the invention enables an even dry content and an end product of highand uniform quality. The uniform quality is guaranteed by the fibrousweb being attached during the dewatering process and by double-sidedevaporation and by subjecting the fibrous web only to very slightmechanical stress. An even dry content in the end product is ensuredsimply by a number of IR heating units on the last drying cylinders ofthe drying section.

DESCRIPTION OF THE FIGURES

In the following the invention will be described by means of a preferredembodiment with reference to the attached drawing, in which

FIG. 1 shows a part of the drying section in a conventional papermachine,

FIG. 2 is a drying section according to the invention, and

FIG. 3 is a partial enlargement of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a part of a drying section in a conventional paper machine.A paper web 1, or other fibrous web, is conveyed at a speed of about 20m/s, supported against a wire 2, with the paper web lying between thewire and a drying cylinder 3. Air blow devices 5 for blowing against thepaper web 1 are arranged by an outlet nip 4 of the drying cylinder.Blowing air accelerates the dewatering of the paper web 1. After thedrying cylinder 1, the paper web is conveyed over cylinders 6 and 7. Asa whole, the drying section comprises several dozens of drying cylinders3.

FIG. 2 shows a part of a drying section of the invention. The dryingsection comprises a drying cylinder 8 on which a paper web 9, or otherfibrous web, such as a cellulose or cardboard web, is conveyed. Thepaper web 9 is conveyed wirelessly. Reference numerals 29 and 32 denoteturning cylinders which may have a conventional construction.

The drying cylinder 8 is porous such that it allows moisture to beconveyed from its shell surface 10 against its inner surface 11. Thedrying cylinder 8 is porous because it is made of a sintered material.The material is suitably metal or ceramic or a mixture thereof. It isalso feasible that the porosity is provided by a perforated metalcylinder. The material the drying cylinder 8 is made from has preferablya relatively high heat conductivity.

Heating means comprising a plurality of heating units 12 are arranged ata distance from the shell surface 10 of the drying cylinder along thelength of the drying cylinder 8.

The heating units 12 are based on heating by electromagnetic waves, asuitable implementation very likely being IR waves with gas as theenergy source. Apertures (not shown) in the heating units allow heatfrom the flame to radiate without hindrance against the upper surface ofthe paper web 9. This provides high heat transfer intensity and highefficiency upon heating of the paper web 9. The paper web 9 isconsiderably heated despite its very high speed, 25 m/s, for example.

The heating units 12 are placed so as to follow the shape of the dryingcylinder 8 such that a first air flow conduit 13 is formed between theheating units and the paper web 9. In FIG. 2, the heating units 12extend along about 180° of the periphery of the drying cylinder 8, butcould be contemplated to extend within a significantly narrower or widerarea. In practice, a usable interval could be for example 40 to 340° ofthe periphery of the drying cylinder 8. To provide effective heatsupply, heating units that are arranged along an area of at least 100°of the periphery of the heating cylinder are, however, preferred. Theheight of the flow conduit 13 may be suitably 2 to 200 mm. An air gap 14in the longitudinal direction of the drying cylinder 8 is arrangedbetween adjacent heating units 12.

The temperature of the air in the flow conduit 13 is between 200 and400° C. such that the temperature in the rightmost heating unit in FIG.2, for example, is several dozens of degrees, typically about 100° C.higher than the temperature in the leftmost heating unit in FIG. 2.

FIG. 2 shows that a casing 15 encircles the heating units 12 and theflow conduit 13. The casing 15 is provided with an inlet 16 for supplyair from a blow device, and an outlet 17 for discharged air having ahigher moisture content. Reference numeral 41 denotes an inner coverthrough which air is allowed to flow. To this end, the cover 41 isprovided with a hole. The use of a porous cover 41 is feasible. Air isblown in and guided as shown by arrows A and B in FIG. 3. Arrows Adenote air blown above and through the gaps 14, and arrow B denotes airwhich, because of the shape and construction of the casing 15, is blownalong the paper web 9 from a blockage 18 at one end of the first flowconduit 13. Consequently, the lateral blow, which provides a humid airflow sweeping lengthwise along the surface of the paper web 9, isprovided in a direction opposite to the movement of the paper web andthe drying cylinder 8. Thus, the blowing device, the casing 15 and thecover 41 may be said to form both a through-blowing device and a lateralblowing device.

Part of the air discharged from the outlet 17 can be used for heatingthe air supplied to the inlet 16. This provides better efficiency.

Within the drying cylinder 8 is arranged an air flow which sweeps alongthe inner surface 11 of the drying cylinder in a direction opposite tothe direction of movement of the drying cylinder. The air flow isprovided by a cylinder-like air guiding device 20 having a centrallongitudinal conduit 39. The air guiding device 20 forms between itsouter periphery and the inner surface 11 of the drying cylinder a flowconduit 21 for air. The flow conduit 21 extends along the length of thedrying cylinder 8 and about 200° of its periphery. However, the flowconduit 21 may extend along a significantly shorter or longer distancealong the periphery of the drying cylinder 8. The air is supplied to theflow conduit 21 from an air inlet 22 arranged in the central conduit 39of the air guiding device 20, via a gap 23 extending in the longitudinaldirection of the air flow device (and the drying cylinder 8).

Consequently, the air which is guided by the air guiding device 20, issupplied laterally to the drying cylinder 8.

Because the air inlet 22 is coupled to the outlet 17 from which heatedair flows, the air in the flow conduit 21 is heated to a temperature of100 to 300° C., preferably 150 to 200° C. The difference between thetemperatures of the air in the flow conduits 13 and 21 is 50 to 100° C.,such that the temperature in the flow conduit 21 in the proximity of theend 18 is about 200° C., and the temperature in the flow conduit in theproximity of the end 19 is about 150° C.

The air is guided from the air inlet 22 along a gap 24 extending in thelongitudinal direction of the air guiding device 20 towards the centreof the air guiding device, the centre being provided with an air outlet25 for humid air.

The heat of the humid air discharged from the air outlet 25 is utilizedin initial blowing inside the casing 15.

The pressure in the flow conduit 21 is arranged somewhat lower than thepressure in the flow conduit 13. Consequently, not only moisture isabsorbed through the porous drying cylinder 8, this being illustrated byarrows C, but the paper web 9 also remains attached against the dryingcylinder 8 such that no supporting wire is required, cf. the wire 2 inFIG. 1. Wireless guide also avoids severe mechanical stress on the paperweb (or other fibrous web). The difference between the pressures in theflow conduits 13 and 21 is between 0.1 and 60 kPa, preferably between 2and 4 kPa. A slight overpressure is present in the flow conduit 13 and aslight negative pressure in the flow conduit 21.

Other heating units 30, which are also based on heating byelectromagnetic waves, preferably infrared rays, are arranged below thedrying cylinder B. These are also encircled by a casing 26 comprising aninlet 27 and an outlet 28. Reference numeral 42 denotes a coverresembling the cover 41. An air gap 31, like the gap 14 for the heatingunits 12, is arranged between adjacent heating units 30 in thelongitudinal direction of the drying cylinder 8.

There is no fibrous web between the heating units 30 and the shell ofthe drying cylinder 8. The heating units act to heat the drying cylinder8 and to discharge humid air from its surface. However, this indirectlyresults in more rapid dewatering of the fibrous web 9.

Reference numeral 40 denotes an inlet nip having ventilation. The inletnip 40 is provided by negative pressure means 25 for keeping the fibrousweb 9 pressed against the drying cylinder. The negative pressure meanscan be for instance suction devices (not shown), with which a negativepressure is provided inside the inner surface 11 of the drying cylinder8, the negative pressure being lower than the one acting in the flowconduit 21. Such suction devices and negative pressure means are easilyachieved by a person in the art, and they are therefore not described inany greater detail herein. Nip ventilation, i.e. suction of air from theinlet nip to the drying cylinder 8, can be arranged in the inlet nip.

Overpressure means 39 for freeing the fibrous web 9 from the dryingcylinder 8 are arranged in the outlet nip of the drying cylinder 9. Theoverpressure means are typically air jets (not shown) which blow againstthe inner surface 11 of the drying cylinder 8. The overpressure means 39see to it that a pressure which is higher than the air pressure in theinlet nip between the drying cylinder 8 and the turning cylinder 32 actsinside the inner surface 11 of the drying cylinder 8. Nip ventilationmay also be arranged in this nip, i.e. suction of air from the inlet nipto the turning cylinder 32. The air guiding device 20 is provided withtwo non-pressurized zones 33, 34. A pressure advantageous to thedurability may act in these zones. In other words, the pressure may beequal to or different from the pressure in the inlet 22.

Reference numeral 35 denotes a succeeding drying cylinder in the dryingsection of the invention.

When the drying section is used and a web break occurs, there are twoalternative procedures:

1) the web is always run through the entire drying section,

2) the web is ‘cut’ at each turning cylinder 29, 32, which also havewhat is known as a web break sequence with overpressure blowing (cf. theoverpressure means 25). The turning cylinders are arranged to movedownwards. A non-pressurized zone 36, an overpressure zone 37 and anegative pressure and overpressure zone 38 act in the turning cylinders29, 32. In web break, overpressure is activated in the middle segment ofthe overpressure zone 38.

The invention has been described above only by means of an example, andit is therefore pointed out that the details of the invention may varyin a variety of ways within the scope of the attached claims. Theheating units 12, 30 may use electricity as the energy source instead ofgas; but, nevertheless, the heating units are based on heating byelectromagnetic waves having a wavelength activating water molecules.The number of heating units may vary (from one to several), as may thetemperature and pressure in the flow conduits 13, 21. The shape of theblow devices 15, 26 outside the drying cylinder 8 and that of the airguiding device 20 inside the drying cylinder 8 may vary. The number ofblow devices outside the drying cylinder may vary (from one to several).The air guiding device may comprise a plurality of arbitrarilypressurized zones for providing the desired differences in pressure,with the best result in mind. The number of drying cylinders may varyand they may be combined with conventional drying cylinders orconventional cylinder dryers.

What is claimed is:
 1. A method of dewatering a fibrous web (9) in adrying section, preferably a paper web, cellulose web or cardboard webin a paper machine, in which method the fibrous web is heated fordecreasing the moisture content thereof, the drying section comprising anumber of drying cylinders (8), by which an inlet and outlet nip areformed when running the fibrous web, the drying section comprisingdewatering devices for decreasing the moisture content of the fibrousweb in which method the fibrous web (9) is guided against a porousdrying cylinder (8), characterized by guiding the fibrous web (9)wirelessly against the porous drying cylinder (8), heating the fibrousweb (9) within the drying section by radiation heat activating watermolecules and directed towards the fibrous web and the drying cylinder(8), and providing by lateral blowing a humid airflow which sweeps inthe machine direction along the surface of the fibrous web (9).
 2. Adrying section for dewatering a fibrous web (9), the drying sectionpreferably being intended to dry fibrous webs in a paper machine, suchas paper webs, cellulose webs or cardboard webs, and the drying sectioncomprising a number of drying cylinders (8), by which an inlet andoutlet nip are formed when running the fibrous web, the drying sectioncomprising dewatering devices for decreasing the moisture content of thefibrous web, the dewatering devices comprising a blow device (15) forblowing air in the proximity of the surface of the fibrous web, and aporous drying cylinder (8) against which the fibrous web (9) is arrangedto be guided, characterized by wireless guiding of the fibrous web (9)against the porous drying cylinder (8), and the dewatering devicescomprising a heating unit (12) arranged within the drying section and ata distance from the shell surface of the porous drying cylinder (8) anddirected towards the fibrous web (9) and drying cylinder, for heatingthe fibrous web, the heating unit being based on heating with rayshaving a wavelength which activates water molecules, and a blow device(15) comprising lateral blow devices arranged to provide a humid airflowwhich sweeps lengthwise along the surface of the fibrous web (9).
 3. Adrying section as claimed in claim 2, characterized in that the lateralblow devices are arranged to blow in a direction opposite to themovement of the fibrous web (9).
 4. A drying section as claimed in claim2, characterized in that the heating unit (12) is based on gas energy,the flame heat being guided without hindrance against the fibrous web(9).
 5. A drying section as claimed in claim 2, characterized in thatthe heating unit is arranged at a distance from the shell surface (10)of the drying cylinder (8) such that a first flow conduit (13) for saidhumid air flow forms between the heating unit and the drying cylinder.6. A drying section as claimed in claim 5, characterized in that theblow device comprises a casing (15) containing the heating units (12)and the first flow conduit (13).
 7. A drying section as claimed in claim2, characterized by at least two heating units (12) arranged along theshell surface (10) of the drying cylinder (8) at a distance from eachother such that an air gap (14) forms between the heating units.
 8. Adrying section as claimed in claim 7, characterized by a plurality ofheating units (12), arranged along the shell surface (10) of the dryingcylinder (8), between which air gaps (14) are formed, respectively, theblow device comprising through-blowing devices for blowing humid airthrough the air gaps (14) against the fibrous web (9).
 9. A dryingsection as claimed in claim 8, characterized in that the heating units(12) are arranged at a distance from the fibrous web (9) along thedrying cylinder (8) at least mainly to follow the shape of a circulararc such that said first flow conduit (13) is formed between the heatingunits and the fibrous web.
 10. A drying section as claimed in claim 9,characterized in that the heating units (12) are arranged along at least100° of the periphery of the drying cylinder (8).
 11. A drying sectionas claimed in claim 9, characterized in that the casing (15) is arrangedto guide the air via the air gaps (14) and along the first flow conduit(13).
 12. A drying section as claimed in claim 2, characterized in thata second flow conduit (21) for air flow is arranged under the shell (10)of the drying cylinder (8), the flow conduit being arranged to guide hotand humid air along the cylindrical inner surface (11) of the dryingcylinder.
 13. A drying section as claimed in claim 12, characterized inthat the drying cylinder (8) is provided with an air inlet (22) and anair outlet (25) for air to and from the second flow conduit (21),respectively, the air pressure in the air inlet and the second flowconduit being arranged to be lower than the air pressure in the firstflow conduit (13) to achieve a pressure difference and material transferfrom the lower surface of the fibrous web (9) to the second flow conduitthrough the drying cylinder (8).
 14. A drying section as claimed inclaim 13, the through-blowing devices comprising an outlet (17),characterized in that the outlet (17) is coupled to the air inlet (22)in the drying cylinder (8) for discharging heated humid air from theoutlet to the air inlet.
 15. A drying section as claimed in claim 13,characterized in that the air inlet (22) is arranged close to the middleof a cylindrical air guiding device (20) having a central longitudinalconduit (39) and an outer diameter smaller than the inner diameter ofthe drying cylinder (8) for providing the second flow conduit (21), theair inlet being located inside the central conduit.
 16. A drying sectionas claimed in claim 13, characterized in that overpressure means (39)are arranged at the outlet nip of the drying cylinder (8) for freeingthe fibrous web (9) from the drying cylinder.